Process for Decontamination of Hazardous Sulfur Compounds in Oilfield Produced Waters

ABSTRACT

A system and method treat oilfield produced water by two-stages of hazardous sulfide treatments. In an embodiment, a two-stage oil and gas field produced water treatment system includes an oil removal vessel. The oilfield produced water is introduced to the oil removal vessel. The oil removal vessel removes a portion of the hydrocarbons from the oilfield produced water to provide a reduced oil produced water. In addition, the system includes an iron sponge. The reduced oil produced water is introduced to the iron sponge, and the iron sponge removes a portion of the hazardous sulfides from the reduced oil produced water to provide a reduced sulfide produced water. The system also includes a stabilized sodium percarbonate solution. The stabilized sodium percarbonate solution is mixed with the reduced sulfide produced water to remove a portion of the hazardous sulfides from the reduced sulfide produced water to provide a treated produced water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application that claims thebenefit of U.S. Application Ser. No. 61/413,640 filed on Nov. 15, 2010,which is incorporated by reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of sour water treatment andspecifically to decontamination of hazardous sulfur compounds inoilfield produced water with a two-stage treatment process using an ironsponge and a stabilized sodium percarbonate solution.

2. Background of the Invention

Water returning from producing oilfields (e.g., oilfield produced water)may include hydrogen sulfide, mercaptans, and other hazardous sulfidessuch as disulfides. This oilfield produced water may originate from thenative formation or may be water returned after injection for wellstimulation activity. Typically, this oilfield produced water isreturned to the formation or treated to reduce hazardous sulfides to amanageable level. Various regulatory agencies may also impose strictguidelines on allowable hazardous sulfides content prior to disposal.Methods have therefore been developed to treat such oilfield producedwater.

Conventional water treatment methods include using solid or liquidhydrogen sulfide scavengers, oxidizing chemicals, or physical separationmethods such as membrane technology or activated carbon. Solidscavengers such as zinc oxide or iron oxide sponges are typically usedfor complexing hydrogen sulfide in solid form. Liquid scavengers such astriazine or acrolein are typically used for complexing hydrogen sulfidein liquid form. Oxidizing chemicals typically include nitrites (i.e.,sodium nitrite), hypochlorite, potassium or sodium permanganate.Oxidizing chemicals are conventionally used to convert hydrogen sulfideto more harmless forms of sulfur such as water-soluble thiosulfate orsulfate.

Drawbacks to such conventional treatment methods include that whileliquid scavengers may be very cost-effective and may tie up hydrogensulfide as water-soluble compounds that are discharged to wastewatertreatment facilities, such facilities are typically inaccessible tooilfield applications. Direct discharge of the treated effluent maytherefore be problematic. Further drawbacks include that while oxidizingchemicals may irreversibly convert hydrogen sulfide to harmless watersoluble forms of sulfur that may be compatible with effluent discharge,the chemicals themselves may pose significant impact to the environment.Additional drawbacks include that permanent forms reaction solids suchas solid manganese dioxide as a reaction product with hydrogen sulfide.Drawbacks to permanganate also include that it costs more per pound andmay not be permitted for disposal without removal of the reactionsolids. Permanganate may also be dangerous as any residual hydrocarbonin the water may invoke an exothermic reaction, which is also a drawbackto hypochlorite. In addition, while nitrite chemicals may be useful ateliminating hydrogen sulfide, heat may be required for activation (i.e.,typically heat accelerates a rather slow oxidation of hydrogen sulfide),which may produce another hazardous chemical (e.g., ammonia) as areaction product. Moreover, costs are typically higher for oxidizingchemicals than for liquid scavenger chemicals. Drawbacks to zinc saltsand oxides include that they may be quite cost prohibitive and produce asolid sulfide-containing residue that may regenerate hydrogen sulfideupon acidification, which may also require a disposal cost. Triazinesand acroleins, while typically suitable for wastewater treatment plants,may form compounds that make direct disposal of treated waterproblematic from a regulatory standpoint. Drawbacks to activated carboninclude disposal, which may be a costly option for large amounts ofhydrogen sulfide.

Iron sponges have been developed to treat gas streams for removal ofhydrogen sulfide. Iron sponges, typically to a lesser extent, have beenused for similar treatments in industrial water applications.SulfaTreat® (a registered trademark of M-I L.L.C.) markets a commercialproduct (SulfaTreat®HC) used for hydrogen sulfide remediation ofnon-potable water. This product may potentially remove most if not allof the hydrogen sulfide from water. Drawbacks include that over time,the efficiency may drop off as the bed is exhausted. Further drawbacksinclude that the product may be less efficient at removal of mercaptans.

Consequently, there is a need for an improved method for decontaminationof hazardous sulfides in oilfield produced waters.

BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS

These and other needs in the art are addressed by a two-stage oil andgas field produced water treatment system for treatment of an oilfieldproduced water. The oilfield produced water comprises hydrocarbons andhazardous sulfides. In embodiments, the two-stage oil and gas fieldproduced water treatment system includes an oil removal vessel. Theoilfield produced water is introduced to the oil removal vessel. Inaddition, the oil removal vessel removes a portion of the hydrocarbonsfrom the oilfield produced water to provide a reduced oil producedwater. The two-stage oil and gas field produced water treatment systemalso includes an iron sponge. The reduced oil produced water isintroduced to the iron sponge. The iron sponge removes a portion of thehazardous sulfides from the reduced oil produced water to provide areduced sulfide produced water. The two-stage oil and gas field producedwater treatment system further includes a stabilized sodium percarbonatesolution. The stabilized sodium percarbonate solution is mixed with thereduced sulfide produced water to remove a portion of the hazardoussulfides from the reduced sulfide produced water to provide a treatedproduced water.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other embodiments for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent embodiments do not departfrom the spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawing inwhich the FIGURE illustrates an embodiment of a two-stage oilfieldproduced water treatment system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE illustrates an embodiment of two-stage oil and gas fieldproduced water treatment system 5. Two-stage oil and gas field producedwater treatment system 5 includes iron sponge 20 (the first stage) andstabilized sodium percarbonate solution 50 (the second stage). In anembodiment, iron sponge 20 is disposed to react with incoming hydrogensulfide in oilfield produced water 75 to provide a reduced iron sulfidesolution having a non-pyrophoric form of iron sulfide. In embodiments,iron sponge 20 provides for complexation of a majority of the hydrogensulfide component in oilfield produced water 75. In some embodiments,stabilized sodium percarbonate solution 50 is mixed with the remaininghydrogen sulfide in the reduced iron sulfide solution to remove residualhydrogen sulfide by converting the hydrogen sulfide into a salt, whichis water soluble. It is to be understood that oilfield produced water 75refers to water that is a by-product of oil and gas recovery operationsand that contains hydrocarbons, solids (both dissolved and un-dissolvedsolids) and hazardous sulfides. Hazardous sulfides may include anyhazardous sulfides typically found in oilfield produced waters. Withoutlimitation, examples of hazardous sulfides include hydrogen sulfides,disulfides, mercaptans, and the like.

In some embodiments as shown, oilfield produced water 75 is provided totwo-stage oil and gas produced water treatment system 5. In anembodiment, oilfield produced water 75 is contained in one or more tanks10. Tank 10 may include any vessel suitable for containing oilfieldproduced water 75. In embodiments, tank 10 is a well test tank in whichoilfield produced water 75 is tested (i.e., tested to determine theamount of oil (e.g., ppm) in water). In an embodiment as shown, oilfieldproduced water 75 is supplied from tank 10 to oil removal vessel 15 inwhich a portion or substantially all of the hydrocarbons in oilfieldproduced water 75 are removed. Oil removal vessel 15 may include anyvessel suitable for removing a portion or substantially all of thehydrocarbons (e.g., oil) from oilfield produced water 75. Withoutlimitation, examples of suitable oil removal vessels 15 include gasflotation units, heat separation units, skimming pits, or anycombinations thereof. In an embodiment, oil removal vessel 15 is a gasflotation unit. A gas flotation unit refers to a unit in which a gas(e.g., carbon dioxide) is bubbled through a water having suspended oil,with the water and oil typically separating at or near the top of theunit. In some embodiments, the gas flotation unit has a skimmer thatremoves the oil from the unit. Oil removal vessel 15 may remove anydesired amount of hydrocarbons from oilfield produced water 75. In anembodiment, oil removal vessel 15 provides reduced oil produced water 80containing between about 20 ppm and about 50 ppm hydrocarbons,alternatively between about 25 ppm and about 35 ppm hydrocarbons, andalternatively about 29 ppm hydrocarbons.

As shown, embodiments include reduced oil produced water 80 from oilremoval vessel 15 being introduced to iron sponge 20. Iron sponge 20 isa vessel containing a sponge material comprising a substrate with atransition metal oxide coating. Any suitable substrate for use with aniron sponge may be used. Without limitation, examples of substratesinclude clay and wood. In an embodiment, the substrate is a claysubstrate. Any suitable transition metal oxide coating may be used. Inan embodiment, the transition metal oxide is zinc oxide, iron oxide, orany combinations thereof. In embodiments, the transition metal oxide isiron oxide. In an embodiment in which the coating is iron oxide, thehydrogen sulfide in reduced oil produced water 80 reacts with the ironoxide to produce iron sulfide. Without limitation, the iron sulfide isproduced on sponge material. Further, without limitation, a commercialexample of the sponge material is SULFATREAT-HC®, which is a registeredtrademark of M-I L.L.C. In some embodiments, pressure is added toreduced oil produced water 80 when introduced to iron sponge 20, whichfacilitates diffusion of reduced oil produced water 80 through ironsponge 20. Iron sponge 20 may be operated to remove any desired portionof the hazardous sulfides. In embodiments, iron sponge 20 is operated toremove between about 80% and about 98% of hazardous sulfides fromreduced oil produced water 80, alternatively between about 85% and about95% of hazardous sulfides from reduced oil produced water 80, furtheralternatively between about 90% and about 98% of hazardous sulfides fromreduced oil produced water 80, and further alternatively about 90% ofhazardous sulfides from reduced oil produced water 80. Iron sponge 20reduces the amount of hazardous sulfides in reduced oil produced water80 to provide reduced sulfide produced water 85.

Stabilized sodium percarbonate solution 50 is added to reduced sulfideproduced water 85. Stabilized sodium percarbonate solution 50 may beadded to reduced sulfide produced water 85 by any suitable means. In anembodiment as shown, stabilized sodium percarbonate solution 50 is addedto reduced sulfide produced water 85 via stabilized sodium percarbonatesolution valve 25. In an embodiment, stabilized sodium percarbonatesolution 50 includes sodium percarbonate and a liquid solution thatincludes water, a stabilizer, and an acid. In embodiments, stabilizedsodium percarbonate solution 50 has a strength retention from about 90%to about 99%, alternatively from about 92% to about 97%, andalternatively about 96%. It is to be understood that strength retentionrefers to the maintenance of active sodium percarbonate in solution. Anystabilizer suitable for stabilizing the sodium percarbonate solution(e.g., the solution of sodium percarbonate and the liquid) may be used.In embodiments, the stabilizer is a peroxide stabilizer. In someembodiments, the stabilizer includes nitrilotrimethylenephosphonate(NTMP); tartrazine; 1-hydroxyethylidene-1, 1 diphosphonic acid; or anycombinations thereof. In an embodiment, the stabilizer comprises NTMP,tartrazine, or any combinations thereof. In some embodiments, thestabilizer is tartrazine. The acid may be any acid suitable to bring thesodium percarbonate to a neutral or desired pH level. In an embodiment,the acid comprises citric acid, phosphoric acid, or any combinationsthereof. In some embodiments, the acid is phosphoric acid. In anembodiment, the liquid solution contains from about 0.01 wt. % to about0.5 wt. % of peroxide stabilizer (i.e., tartrazine), alternatively fromabout 0.01 wt. % to about 0.05 wt. % peroxide stabilizer (i.e.,tartrazine), and alternatively about 0.05 wt. % peroxide stabilizer(i.e., tartrazine). The liquid solution contains a sufficient amount ofacid to maintain the pH of the liquid solution below about 7.0 pH,alternatively below about 6.5 pH, and alternatively from about 5.5 pH toabout 6.5 pH. In an embodiment, the acid is phosphoric acid. In suchembodiments, the phosphoric acid may be any suitable percent phosphoricacid. For instance, the phosphoric acid may be 75% phosphoric acid or85% phosphoric acid. The liquid solution may contain any suitable amountof the acid to maintain the desired pH level. In embodiments, the liquidsolution contains from about 4.0 vol. % phosphoric acid to about 8.0vol. % phosphoric acid, alternatively from about 5.0 vol. % phosphoricacid to about 7.0 vol. % phosphoric acid, and alternatively about 6.0vol. % phosphoric acid. In embodiments in which the acid is citric acid,the liquid solution contains citric acid in amounts that maintain pH atlevels about commensurate with those in phosphate-buffered solutions.Without limitation, a commercial example of stabilized sodiumpercarbonate solution 50 is SWT-K13™, which is a trademark of UnitedLaboratories International, LLC.

In embodiments, a sufficient amount of stabilized sodium percarbonatesolution 50 is added to reduced sulfide produced water 85 to remove aportion or substantially all of the hazardous sulfides in reducedsulfide produced water 85. In embodiments, a sufficient amount ofstabilized sodium percarbonate solution 50 is added to reduced sulfideproduced water 85 to reduce the amount of hazardous sulfides instabilized sodium percarbonate solution 50 to between about 1 ppm andabout 20 ppm, alternatively between about 1 ppm and about 10 ppm, andalternatively between about 1 ppm and about 5 ppm, further alternativelyless than about 4 ppm, and alternatively less than about 1 ppm. Aftermixing with reduced sulfide produced water 85 to reduce the amount ofhazardous sulfides, embodiments of two-stage oil and gas field producedwater treatment system 5 include providing reduced sulfide producedwater 85′ to formation material removal device 35. In alternativeembodiments (not shown), reduced sulfide produced water 85′ is providedto disposal 55 or an alternative destination.

In an embodiment as shown in the Figure, formation material removaldevice 35 removes a portion or substantially all of the formationmaterials and other solids from reduced sulfide produced water 85′.Formation materials refer to seashells, sand, rocks, and the like.Formation material removal device 35 may include any device or processsuitable for removing solids from a liquid. Without limitation, examplesof suitable formation material removal devices 35 include cartridgefilters, bag filters, screen filters, or any combinations thereof. In anembodiment, formation material removal device 35 comprises cartridgefilters. Any cartridge filters suitable for removing formation materialsand other solids may be used. In embodiments, cartridge filters comprisecorrugated materials. In some embodiments, pressure is added to reducedsulfide produced water 85′. In an embodiment as shown, pump 30 providespressure to reduced sulfide produced water 85′ before introduction toformation material removal device 35.

As shown in the Figure, embodiments of two-stage oil and gas fieldproduced water treatment system 5 include introducing reduced sulfideproduced water 85′ to hydrocarbon removal device 40. In embodiments asshown, reduced sulfide produced water 85′ is introduced to hydrocarbonremoval device 40 after treatment by formation material removal device35. Without limitation, reduced sulfide produced water 85′ is treated byformation material removal device 35 prior to treatment by hydrocarbonremoval device 40 because the removal of remaining solids by formationmaterial removal device 35 facilitates the operation of hydrocarbonremoval device 40. In embodiments, hydrocarbon removal device 40 removestrace hydrocarbons from reduced sulfide produced water 85′. Hydrocarbonremoval device 40 includes any device suitable for removing tracehydrocarbons from reduced sulfide produced water 85′. Withoutlimitation, examples of suitable hydrocarbon removal devices 40 includecarbon filters, clay, sawdust, or any combinations thereof. In anembodiment, hydrocarbon removal device 40 comprises carbon filters. Anysuitable type of carbon filter may be used. In some embodiments, thetype of carbon filter used may be selected based on the efficiency ofoil removal vessel 15. In embodiments, hydrocarbon removal device 40treats reduced sulfide produced water 85′ to provide treated producedwater 90 having between about 1 ppm and about 100 ppm hydrocarbons,alternatively between about 1 ppm and about 50 ppm hydrocarbons.

In an embodiment of operation of two-stage oil and gas produced watertreatment system 5, oilfield produced water 75 is introduced to tank 10(e.g., well test tank). In such an embodiment, the composition ofoilfield produced water 75 is determined. From tank 10, oilfieldproduced water 75 is introduced to oil removal vessel 15 (e.g., gasflotation unit). Oil removal vessel 15 is operated to remove a desiredamount of hydrocarbons from oilfield produced water 75 and therebyprovide reduced oil produced water 80, which is then introduced to ironsponge 20. Iron sponge 20 is operated to remove a desired amount ofhazardous sulfides from reduced oil produced water 80. In an embodiment,the effectiveness of iron sponge 20 at removing hazardous sulfides isdetermined. In embodiments, the effectiveness is determined by spongemeasurement device 95, which measures reduced sulfide produced water 85content before addition of stabilized sodium percarbonate solution 50.Sponge measurement device 95 may be any device suitable for measuringthe content of a liquid. In embodiments, sponge measurement device 95 isa sensor. Any suitable sensor may be used. In an embodiment, the sensoris a membrane/lead acetate sensor. In embodiments, the analysis bysponge measurement device 95 is automatic. In alternative embodiments,the analysis by sponge measurement device 95 is manual. For instance,such manual analysis may be by silver titration methods. In embodiments,the content measurement of reduced sulfide produced water 85 by spongemeasurement device 95 includes measurements (i.e., amount, wt. %, andthe like) of hazardous sulfides in reduced sulfide produced water 85. Inan embodiment, such content measurement is used to determine the amountof stabilized sodium percarbonate solution 50 to add. For instance, inembodiments, two-stage oil and gas field produced water treatment system5 has a target hazardous sulfide content in reduced sulfide producedwater 85′. To achieve such target, when sponge measurement device 95determines that the hazardous sulfide content of reduced sulfideproduced water 85 exiting iron sponge 20 is greater than such target, asufficient amount of stabilized sodium percarbonate solution 50 is addedto reduced sulfide produced water 85 to reduce the hazardous sulfidecontent to about at or below the desired target.

In embodiments, the amount of hazardous sulfides in reduced sulfideproduced water 85 is determined (i.e., by the determination in tank 10of the content of oilfield produced water 75). Embodiments of two-stageoil and gas field produced water treatment system 5 include comparingthe hazardous sulfide content of reduced sulfide produced water 85exiting iron sponge 20 to the hazardous sulfide content of reduced oilproduced water 80 entering iron sponge 20 to determine and monitor theeffectiveness of iron sponge 20 in removing hazardous sulfides. In anembodiment, two-stage oil and gas field produced water treatment system5 includes a threshold for removal of hazardous sulfides by iron sponge20. In embodiments, such threshold includes a determination of thepercentage of hazardous sulfides removed by iron sponge 20. Since thehazardous sulfide content of reduced oil produced water 80 entering ironsponge 20 and that of reduced sulfide produced water 85 exiting ironsponge 20 are determined, the amount and percent removal by iron sponge20 may be determined. The threshold may be any desired removal percent.In embodiments, the threshold is removal of about 95 percent or higher,alternatively about 90 percent or higher, and alternatively about 50percent or higher of hazardous sulfides. In an embodiment, the thresholdis removal of about 90 percent or higher of hazardous sulfides. When thepercentage of hazardous sulfides removed is below the threshold, ironsponge 20 may be taken off-line, with stabilized sodium percarbonatesolution 50 added to reduced oil produced water 80 in increased amountsto remove the amount of hazardous sulfides that were removed by ironsponge 20 and to achieve the target. In such an embodiment, with ironsponge 20 off-line, stabilized sodium percarbonate solution 50 isperforming both stages of hazardous sulfide removal. While off-line,maintenance is performed on iron sponge 20. When the maintenance iscomplete, iron sponge 20 is placed back in-line to reduce the hazardoussulfide content in reduced oil produced water 80 (i.e., the first stage)with stabilized sodium percarbonate solution 50 reducing the hazardoussulfide content of reduced sulfide produced water 85 (i.e., the secondstage).

Maintenance on iron sponge 20 may be accomplished by any suitablemethod. In an embodiment, maintenance includes a sponge replacementmethod. The sponge replacement method includes replacing the spongematerial. In such an embodiment, the sponge material is removed fromiron sponge 20 and sent to a desired location such as a landfill. Freshsponge material may then be added to iron sponge 20. In anotherembodiment, the maintenance is a limited regeneration method. In such anembodiment of a limited regeneration method, air is introduced to ironsponge 20 with the air contacting the sponge material with iron sulfideon its exterior. The air regenerates the iron sulfide to form iron oxideand solid sulfur. In some embodiments, the maintenance is a totalregeneration method. In such embodiments of a total regeneration method,stabilized sodium percarbonate solution 50 is introduced to iron sponge20 with stabilized sodium percarbonate solution 50 contacting the spongematerial with iron sulfide on its exterior. Stabilized sodiumpercarbonate solution 50 regenerates the iron sulfide to water and ironoxide. In some embodiments, repeated maintenance may include the limitedregeneration method until the sulfur coats a portion or all of thesurface of the sponge material. Without being limited by theory, sulfurcoating the surface (i.e., the iron oxide surface) may limit conversionof the hazardous sulfides (i.e., hydrogen sulfide conversion). In suchinstances when sulfur coats a portion or all of the surface, the spongereplacement method and/or the total regeneration method may then beused.

Further, in an embodiment of operation of two-stage oil and gas fieldproduced water treatment system 5, reduced sulfide produced water 85′ isprovided to formation material removal device 35 (i.e., cartridgefilter) via pump 30. Solids are removed by formation material removaldevice 35, and reduced sulfide produced water 85′ is then introduced tohydrocarbon removal device 40 (i.e., carbon filter) in which tracehydrocarbons are removed from reduced sulfide produced water 85′ toprovide treated produced water 90. In an embodiment, the content oftreated produced water 90 is determined. If the content of treatedproduced water 90 is above a desired final target content, embodimentsinclude recycling treated produced water 90. The final target has amaximum amount (i.e., in ppm) of hydrocarbons (e.g., oil) and/orhazardous sulfides (e.g., hydrogen sulfide). In embodiments, thehazardous sulfide content of treated produced water 90 is determined byrecycle measurement device 100, which measures treated produced water 90hazardous sulfide content. Recycle measurement device 100 may be anydevice suitable for measuring the content of a liquid. In embodiments,recycle measurement device 100 is a sensor. Any suitable sensor may beused. In an embodiment, the sensor is a membrane/lead acetate sensor. Inembodiments, the analysis by recycle measurement device 100 isautomatic. In alternative embodiments, the analysis by recyclemeasurement device 100 is manual. For instance, such manual analysis maybe by silver titration methods. Any suitable device (not illustrated)may be used to determine the hydrocarbon content of treated producedwater 90 such as, without limitation, infrared sensors, ultra violetturbidity measurements, and the like.

In an instance in which the content of treated produced water 90 isdetermined by recycle measurement device 100 to have a hazardous sulfidecontent greater than the final target for hazardous sulfides,embodiments of two-stage oil and gas field produced water treatmentsystem 5 include recycling treated produced water 90 as recycle stream60. In such embodiments, recycle stream 60 is recycled and re-injectedwith stabilized sodium percarbonate solution 50 and mixed to be includedwith reduced sulfide produced water 85. In an embodiment, recycle stream60 is re-injected at stabilized sodium percarbonate solution valve 25with stabilized sodium percarbonate solution 50. In alternativeembodiments, recycle stream 60 is re-injected into iron sponge 20. Inembodiments, whether recycle stream 60 is recycled to iron sponge 20 orto re-injection with stabilized sodium percarbonate solution 50 isdetermined based upon how much higher the hazardous sulfide content isthan the final target. In embodiments, in instances in which thehazardous sulfide content is greater than 100 ppm above the finaltarget, recycle stream 60 is recycled to iron sponge 20. In suchembodiments, in instances in which the hazardous sulfide content in lessthan 100 ppm above the final target, recycle stream 60 is recycled tore-injection with stabilized sodium percarbonate solution 50.

In an instance in which the content of treated produced water 90 isdetermined to have a hydrocarbon content greater than the final targetfor hydrocarbons, embodiments of two-stage oil and gas field producedwater treatment system 5 include recycling treated produced water 90 asrecycle stream 60. In such embodiments, recycle stream 60 is recycledand re-injected to oil removal vessel 15 or hydrocarbon removal device40. In embodiments, whether recycle stream 60 is recycled to oil removalvessel 15 or to hydrocarbon removal device 40 is determined based uponhow much higher the hydrocarbon content is than the final target. Inembodiments, in instances in which the hydrocarbon content is greaterthan 100 ppm above the final target, recycle stream 60 is recycled tooil removal vessel 15. In such embodiments, in instances in which thehydrocarbon content in less than 100 ppm above the final target, recyclestream 60 is recycled to hydrocarbon removal device 40.

In some embodiments, two-stage oil and gas field produced watertreatment system 5 is automated. It is to be understood that two-stageoil and gas field produced water treatment system 5 is not limited tothe embodiments as shown in the Figure. In alternative embodiments (notillustrated), two-stage oil and gas field produced water treatmentsystem 5 does not have tank 10. In other alternative embodiments (notillustrated), two-stage oil and gas field produced water treatmentsystem 5 does not have pump 30, formation material removal device 35,and/or hydrocarbon removal device 40. In alternative embodiments (notillustrated) in which two-stage oil and gas field produced watertreatment system 5 does not have pump 30, formation material removaldevice 35, and hydrocarbon removal device 40, treated produced water 90is reduced sulfide produced water 85′. In other alternative embodiments(not illustrated), two-stage oil and gas field produced water treatmentsystem 5 does not have oil removal vessel 15. In such alternativeembodiments, oilfield produced water 75 is introduced to iron sponge 20,and/or oilfield produced water 75 has hydrocarbons removed elsewhere toprovide reduced oil produced water 80, which is then introduced to ironsponge 20.

To further illustrate various illustrative embodiments of the presentinvention, the following example is provided.

EXAMPLE

This example was to measure the reaction exotherm between a sour waterand a stabilized sodium percarbonate solution. SWT-K13™ was thestabilized sodium percarbonate solution used.

A sour water assayed at 10,200 ppm hydrogen sulfide was used. 11 mls ofthis sour water was diluted to 100 mls in a capped bottle. 2 mls of oil(motor oil) was added to the 100 mls of sour water. 1 ml of a dispersantwas added to disperse the motor oil. The SWT-K13™ was assayed at 9.60%active.

100 mls of sour water was treated with 33 mls of SWT-K13™ by placementin a constant temperature bath at 60° C. After an hour, both liquids hadreached a stable temperature of 58° C. The SWT-K13™ was added to thesour water, and a temperature rise was monitored. The temperature of thetreated sour water rose from 58° C. to 62° C. within two minutes andthereafter remained stable.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations may be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A two-stage oil and gas field produced water treatment system fortreatment of an oilfield produced water, wherein the oilfield producedwater comprises hydrocarbons and hazardous sulfides, comprising: an oilremoval vessel, wherein the oilfield produced water is introduced to theoil removal vessel, and wherein the oil removal vessel removes a portionof the hydrocarbons from the oilfield produced water to provide areduced oil produced water; an iron sponge, wherein the reduced oilproduced water is introduced to the iron sponge, and wherein the ironsponge removes a portion of the hazardous sulfides from the reduced oilproduced water to provide a reduced sulfide produced water; and astabilized sodium percarbonate solution, wherein the stabilized sodiumpercarbonate solution is mixed with the reduced sulfide produced waterto remove a portion of the hazardous sulfides from the reduced sulfideproduced water to provide a treated produced water.
 2. The two-stage oiland gas field produced water treatment system of claim 1, wherein theiron sponge removes between about 80% and about 98% of the hazardoussulfides from the reduced oil produced water.
 3. The two-stage oil andgas field produced water treatment system of claim 1, wherein thestabilized sodium percarbonate solution comprises a strength retentionfrom about 90% to about 99%.
 4. The two-stage oil and gas field producedwater treatment system of claim 1, wherein the stabilized sodiumpercarbonate solution reduces the hazardous sulfides in the reducedsulfide produced water to between about 1 ppm and about 20 ppm.
 5. Thetwo-stage oil and gas field produced water treatment system of claim 1,further comprising a sponge measurement device, wherein the spongemeasurement device determines the amount of hazardous sulfides in thereduced sulfide produced water.
 6. The two-stage oil and gas fieldproduced water treatment system of claim 5, further comprising a targethazardous sulfide content in the reduced sulfide produced water.
 7. Thetwo-stage oil and gas field produced water treatment system of claim 6,wherein an amount of the stabilized sodium percarbonate solution ismixed with the reduced sulfide produced water to reduce the hazardoussulfide content in the reduced sulfide produced water to about at orabout below the desired target.
 8. The two-stage oil and gas fieldproduced water treatment system of claim 1, further comprising a recyclemeasurement device, wherein the recycle measurement device determinesthe amount of hazardous sulfides.
 9. The two-stage oil and gas fieldproduced water treatment system of claim 1, further comprising a finaltarget comprising a maximum amount of hydrocarbons and/or a maximumamount of hazardous sulfides in the treated produced water.
 10. Thetwo-stage oil and gas field produced water treatment system of claim 9,further comprising recycling the treated produced water when the amountof the hydrocarbons in the treated produced water is greater than themaximum amount of hydrocarbons.
 11. The two-stage oil and gas fieldproduced water treatment system of claim 10, further comprisingrecycling the treated produced water to the oil removal vessel.
 12. Thetwo-stage oil and gas field produced water treatment system of claim 9,further comprising recycling the treated produced water when the amountof the hazardous sulfides in the treated produced water is greater thanthe maximum amount of hazardous sulfides.
 13. The two-stage oil and gasfield produced water treatment system of claim 12, further comprisingrecycling the treated produced water to the iron sponge.
 14. Thetwo-stage oil and gas field produced water treatment system of claim 12,further comprising recycling the treated produced water to re-injectionwith the stabilized sodium percarbonate solution.
 15. The two-stage oiland gas field produced water treatment system of claim 1, furthercomprising a threshold for the removal of hazardous sulfides by the ironsponge, and wherein the iron sponge is taken off-line for maintenancewhen an amount of hazardous sulfides removed by the iron sponge is lessthan the threshold.
 16. The two-stage oil and gas field produced watertreatment system of claim 15, wherein the maintenance comprises a spongereplacement method.
 17. The two-stage oil and gas field produced watertreatment system of claim 15, wherein the maintenance comprises alimited regeneration method comprising introducing air to the ironsponge.
 18. The two-stage oil and gas field produced water treatmentsystem of claim 15, wherein the maintenance comprises a totalregeneration method comprising introducing the stabilized sodiumpercarbonate solution to the iron sponge.
 19. The two-stage oil and gasfield produced water treatment system of claim 1, further comprising aformation material removal device, wherein the formation materialremoval device removes formation materials from the reduced sulfideproduced water.
 20. The two-stage oil and gas field produced watertreatment system of claim 1, further comprising a hydrocarbon removaldevice, wherein the hydrocarbon removal device removes hydrocarbons fromthe reduced sulfide produced water.